Sonotube Calculator
How much concrete goes in a cardboard tube footing: volume per tube by diameter and depth, and 60 or 80 lb bags for one tube or a whole row of footings.
How much concrete fills a tube
A tube footing is just a cylinder, so its concrete is cylinder math: the circle of the tube times its depth. What surprises people is how fast the diameter squares — step from an 8 in tube to a 12 in and the concrete per foot of depth more than doubles, because area grows with the square of the radius. A bag of ready-mix fills a standard slice of that cylinder: an 80 lb bag yields about 0.60 cu ft of concrete, a 60 lb bag about 0.45.
Volume per tube = π × (diameter ÷ 2)² × depth · Bags = total volume (with allowance) ÷ bag yield (0.60 cu ft per 80 lb bag, 0.45 per 60 lb)
Example: six 12 in tubes, 4 ft deep: each tube holds 3.14 cu ft, the row takes 18.85 cu ft in all — 35 bags of 80 lb mix (47 of 60 lb) with the allowance in.
The depth that matters is set by frost: a footing must reach below your local frost line so frozen ground cannot heave it, and most codes also want the tube to bell out or sit on undisturbed soil at that depth. Frost lines run from a few inches in the deep South to 4 ft and beyond in the northern states — it is a local number, set by your climate and code, so confirm it locally before you dig rather than copying a neighbor's depth from a different state. The tubes themselves are cheap insurance against the hole caving or the concrete necking in soft soil; in firm clay some builders pour against the augered hole and skip the tube, but the volume then follows the hole, warts and all.
Frequently asked questions
How deep should a footing tube go?
Below the local frost line — that is the rule everywhere the ground freezes. Frost depth is set by climate and adopted by local code: roughly 12 in in mild states, 36–48 in across the northern tier, deeper in the coldest. Your building department publishes the number for your county; it is the first thing to confirm before choosing a depth.
80 lb or 60 lb bags — which should I buy?
The concrete is the same; the lifting is not. An 80 lb bag yields about a third more per trip (0.60 vs 0.45 cu ft) and usually costs less per cubic foot, but it is a heavy, awkward carry. Many builders mix sizes: 80s for the bulk, a few 60s to finish the count without opening one bag too many.
Should the tube stick up out of the ground?
A few inches, yes — keeping the post base and the concrete's top above grade sheds water and keeps soil (and its moisture) off the wood. A common pattern is the tube cut so its top sits 2–4 in above finished grade, with the concrete domed slightly so water runs off.
Do tube footings need rebar?
A single vertical bar or two is cheap strength and common practice, and many codes ask for it in pier footings. It matters most where uplift (wind on a deck roof or pergola) or frost can work the pier. Local code governs — but one bar per tube is rarely the place to save money.
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